Operation of coal-fired boilers, heavy oil-fired boilers, and combustion furnaces attached to various chemical devices emits exhaust gas containing nitrogen oxides (hereinbelow, abbreviated as NOx). NOx, which are air contaminants that cause photochemical smog and acid rain, have to be removed from exhaust gas before the exhaust gas is emitted outside plants. One method to remove NOx from exhaust gas includes the selective catalytic reduction method. The selective catalytic reduction method is a method for decomposing NOx by reaction of NOx with ammonia using a reduction catalyst to detoxify NOx. The selective catalytic reduction method is widely industrialized as the most economical and effective method.
FIG. 4 exemplifies a configuration of a NOx removal device using the selective catalytic reduction method. In FIG. 4, combustion exhaust gas generated in a boiler 1 passes through a superheater 2 and economizer 3, and arrives at a flue 4 to be introduced into a NOx removal reactor 6. The flue 4 is provided with an ammonia injector 5, which injects ammonia gas necessary for NOx removal reaction into the flue 4. NOx in the combustion exhaust gas are decomposed into nitrogen and water while passing through a catalyst layer 7 placed in the NOx removal reactor 6. Then, the combustion exhaust gas passes through an air heater 8, an electric precipitator 9, and a combustion exhaust gas fan 10 to be emitted from a chimney 11 to the air.
A catalyst layer 7 placed in the NOx removal reactor 6 is mainly composed of a parallel gas flow type catalyst in a grid or plate shape. In the form of the parallel gas flow type catalyst, combustion exhaust gas flows in parallel along a surface of the NOx removal catalyst. Thus, there is an advantage that dust and soot in combustion exhaust gas have few opportunities to come in contact with the surface of the NOx removal catalyst to thereby be slightly deposited on the surface of the NOx removal catalyst. Accordingly, such type of catalyst is widely adopted for NOx removal devices such as coal-fired NOx removal devices and heavy oil-fired NOx removal devices.
The NOx removal catalyst adopted in these NOx removal devices has a substrate of titanium oxide (TiO2). The substrate carries active components such as vanadium pentoxide (V2O5), tungsten oxide (WO3), and molybdenum oxide (MoO3).
Although the aforementioned NOx removal catalyst can attain a high NOx removal performance over a wide range of temperatures, it has a problem that the performance is gradually degraded over a long-term use. The causes of degradation in the NOx removal performance include (1) clogging of gas passage channel by deposition of dust and soot on the surface of the NOx removal catalyst surface, (2) poisoning of the NOx removal catalyst by diffusion of poisoning components in the dust and soot deposited on the surface of the NOx removal catalyst into the NOx removal catalyst, (3) prevention of progression of NOx removal reaction by physical adsorption of a substance contained in fuel to be catalyst poison on the NOx removal catalyst after gasification in a furnace or by chemical reaction of the substance with catalyst components.
The degradation in the NOx removal performance caused by deposition of dust and soot on the surface of the NOx removal catalyst, as (1) or (2) aforementioned, is expected to be suppressed by installation of a dust collector at the combustion exhaust gas inlet of the catalyst layer 7 to reduce the amount of dust and soot arriving at the catalyst layer 7.
In contrast, when the NOx removal catalyst is poisoned by gaseous components as (3) aforementioned, no measures to prevent arrival of poisoning components at the catalyst layer 7 are available now. Thus, durability of the NOx removal catalyst largely depends on types and amounts of toxic substances contained in fuel.
Coal-fired boilers use coal as fuel. Quality of coal varies a great deal depending on the source of the coal, and some coal contains much arsenic. Arsenic is a poisoning component and has a strong action as catalyst poison. When coal containing arsenic in the order of ppm is used as fuel, arsenic deposits on active sites of the NOx removal catalyst to thereby deactivate the active sites in several tens of thousand hours. Accordingly, measures against arsenic are important for installation of a NOx removal facility in a coal-fired boiler and the like.
Arsenic in fuel, the most part of which is gasified when the fuel is burned in a furnace, is present in the form of arsenic trioxide (As2O3). For As2O3 gas, reaction of the equation (1) or (2) is thermodynamically expected to progress in the temperature range of the vicinity of where the NOx removal device is installed.As2O3+O2→As2O5  (I)3CaO+As2O3+O2→Ca3(AsO4)2  (II)
In the equation (I), As2O3 reacts with the surrounding oxygen to change into diarsenic pentoxide in solid state.
In the equation (II), As2O3 reacts with CaO contained in dust and soot to change into calcium arsenate (Ca3(AsO4)2) in solid state.
As2O5 and Ca3(AsO4)2 formed according to the equations (I) and (II) are in solid particle form. Therefore, even if the compounds deposit on the surface of the NOx removal catalyst, the possibility of the compound to be incorporated into the catalyst is low, and influence on the activity of the NOx removal catalyst should be small. However, catalyst deterioration is actually caused by arsenic. In view of above, since the reaction rate of the equation (1) is low, the considerable amount of arsenic is believed to be still present in As2O3 gaseous form also in the vicinity of the catalyst layer 7.
As measures against the deterioration of the catalyst caused by the aforementioned arsenic, a method for preventing deterioration of NOx removal catalyst by installation of an adsorbent-filled layer to adsorb and remove arsenic compounds in the combustion exhaust gas passage upstream of the NOx removal catalyst-filled layer is suggested (see Patent Literatures 1 and 2).
Patent Literatures 3 and 4 suggest a method for wet-washing a NOx removal catalyst with an acid aqueous solution of pH 4 or less and a quaternary ammonium hydroxide.
Patent Literature 5 suggests a method for separating arsenic from a NOx removal catalyst using an inert gas such as Ar, N2, and He containing a reducing agent. In Patent Literature 5, the reducing agent is H2, CO, or CH4, which is contained in the inert gas at 2% (on volume basis). In Patent Literature 5, treatment with the inert gas containing a reducing agent is carried out under the temperature condition of 500° C. or more, and preferably of 700° C. to 900° C.
Patent Literature 6 suggests a method for wash treatment with a multifunctional complex compound after reduction treatment with SO2, CO, H2, CH4, NH3, and the like that are added with HCl.